The present invention relates to ceramic substrates used mainly in the semiconductor industry, and particularly to ceramic substrates suitable for hot plates, electrostatic chucks, wafer probers and so on.
Semiconductors are very important products necessary in various industries. Semiconductor chips are produced, for example, by slicing a silicon monocrystal into a predetermined thickness to produce a silicon wafer, and then forming a plurality of integrated circuits and the like on the silicon wafer.
In a process for manufacturing such semiconductor chips, a silicon wafer put on an electrostatic chuck is subjected to various treatments such as etching and CVD to form conductor circuits, elements and so on. At this time, a corrosive gas as a gas for deposition or a gas for etching is used. Therefore, since it is necessary to protect an electrostatic electrode layer and it is also necessary to induce adsorption power, an electrostatic electrode layer is usually covered with a ceramic dielectric film and the like.
Nitride ceramics have hitherto been used as such a ceramic dielectric film. For example, electrostatic chucks using nitrides such as aluminum nitride are disclosed in JP Kokai Hei 5-8140. Carbon-containing aluminum nitride having an Alxe2x80x94Oxe2x80x94N structure is disclosed in JP Kokai Hei 9-48668.
A process for manufacturing such electrostatic chucks is disclosed in JP Kokoku Hei 6-97677 and so on.
As disclosed, for example, in JP Kokai Sho 62-264638 and JP Kokai Sho 60-261377, such electrostatic chucks made of ceramic are manufactured by the so-called green sheet method wherein a lamination is prepared by laminating green sheets and then a ceramic substrate is produced by firing the lamination.
However, it was found: that an electrostatic chuck using a ceramic produced by the above-mentioned green sheet method has the problem in dispersion of chuck power, that a ceramic heater also has the problem in dispersion of temperature on a wafer treating face; and that a wafer prober has a problem in dispersion of applied voltage of a guard electrode or a ground electrode.
The present invention has been made for solving the above-mentioned problems. The objective of the invention is to provide: a ceramic substrate generating no dispersion of chuck power in case of a ceramic substrate with an electrostatic electrode embedded inside the ceramic substrate (an electrostatic chuck); a ceramic substrate having no temperature difference among positions on its wafer treating face and also being able to evenly heat an object to be heated such as a semiconductor wafer in case of a ceramic substrate with a resistance heating element provided on the surface of the ceramic substrate or inside the ceramic substrate (a hot plate); a ceramic substrate generating no dispersion of a voltage applied to the guard electrode or the ground electrode and being able to remove stray capacitor or noise with certainty in case of a wafer prober with a guard electrode or a ground electrode provided inside the wafer prober(a ceramic substrate).
Under the above-mentioned objective, the inventors of the present invention analyzed the causes for the generation of the above-mentioned problems and have found that a formed conductor layer has dispersion of thickness and thus the dispersion of thickness causes dispersion of chuck power, dispersion of temperature on a wafer treating face and dispersion of voltage applied to electrodes.
Thus, they have finally identified the following. By the adjustment of the dispersion of thickness of the green sheets, or roughness of surface of green sheets used for the preparation of a lamination, within predetermined ranges or, in case of the formation of a conductor layer using a metal foil and the like, by making the thickness of the metal foil and the like even and the like, local dispersion of thickness dependent on portions of a conductor layer of a ceramic substrate to be produced will be reduced with a consequent result of: the elimination of local dispersion of chuck power in case of the manufacture of an electrostatic chuck; the elimination of local dispersion of temperature on a wafer treating face in the manufacture of a hot plate; and the elimination of dispersion of voltage applied to a guard electrode or a ground electrode in the manufacture of a wafer prober.
Furthermore, besides above-mentioned problem, rapid temperature rising and rapid temperature falling are required for hot plates, electrostatic chucks and wafer probers in order to quicken through put, hence, ceramic substrates have another problem, in addition to the above-mentioned problems, that they tend to crack or warp due to such rapid changes in temperature.
However, the inventors have accomplished the present invention through their finding that such a problem can be solved by adjusting the dispersion of thickness of a conductor layer or by adjusting the ratio of the thickness of a conductor layer to that of a ceramic substrate.
That is, the ceramic substrate of the present invention is a ceramic substrate provided with a conductor layer on the surface of the ceramic substrate or inside the ceramic substrate,
wherein: the ratio (t2/t1) of the average thickness of the conductor layer (t2) to the average thickness of the ceramic substrate (t1) is less than 0.1 and; a dispersion of the thickness of the conductor layer to the average thickness of the conductor layer is in a range of xe2x88x9270 to +150%.
In the above-mentioned ceramic substrate, it is desirable that the ceramic substrate is in a disc-shape with a diameter exceeding 150 mm and is also desirable that the thickness of the ceramic substrate is 25 mm or less.
Moreover, in the above-mentioned ceramic substrate, it is desirable that the conductor layer is an electrostatic electrode and that the ceramic substrate functions as an electrostatic chuck.
Furthermore, in the above-mentioned ceramic substrate, it is desirable: that the above-mentioned conductor layer is a resistance heating element; and that the ceramic substrate functions as a hot plate.
Furthermore, in the above-mentioned ceramic substrate, it is desirable that the above-mentioned conductor layer is any of a chuck top electrode a guard electrode and a ground electrode and that the ceramic substrate functions as a wafer prober.
The conventional electrostatic chucks and ceramic heaters have a problem that the use of a green sheet 91 having large dispersion of thickness or having large roughness on its surface as shown in FIG. 15 results in a formation of a conductor layer whose ratio of thickness thereof to a thickness of a ceramic substrate is extremely large, therefore, warp or cracks are generated when rapid temperature increase is conducted.
Furthermore, there also are problems such that when it is used as a hot plate, big local temperature difference is generated in a wafer treating face; that, when it is used as an electrostatic chuck, dispersion of adsorbing power is generated; and that, when it is used as a wafer prober, dispersion of voltage applied is generated.
Such tendencies become particularly noticeable for those in a disc-shape with a diameter exceeding 150 mm. At the times of the filing of JP Kokai Sho 62-264638 and JP Kokai Sho 60-261377, since only those with diameters as small as about 150 mm were required in the market, dispersion of chuck power or temperature difference in a wafer treating face caused little problem.
In addition, since the green sheets used had a thicknesses of about 50 to 150 xcexcm, no serious problem was caused even if green sheets was uneven in thickness.
In the present invention, adjustment is made so that the ratio (t2/t1) of the average thickness (t2) of a conductor layer to the average thickness (t1) of a ceramic substrate becomes less than 0.1; and that a dispersion of the thickness of the conductor layer to the average thickness of the conductor layer is within a range of xe2x88x9270 to +150%.
The dispersion itself is desirably small, but it is not possible to make the dispersion exactly zero because some dispersion will inevitably be caused due to drying of a conductor containing paste, a green sheet and the like. However, even if there is such dispersion and the dispersion does not become exactly zero, adjustment of the thickness of a conductor layer and a ceramic substrate can control the problem that warp or cracks are generated, even when rapid temperature rising is conducted.
In particular, when a ceramic substrate is used as a heater (a hot plate), the fact that the ratio (t2/t1) of the average thickness of a conductor layer (t2) to the average thickness of a ceramic substrate (t1) is less than 0.1 means that the ratio of the thickness of ceramic substrate to the thickness of the resistance heating element becomes large. Therefore, even if some dispersion is caused in the calorific value generated from the resistance heating element, the heat diffuses during its transmission through the inside of the ceramic substrate and thus, the temperature of the wafer treating face is becomes even. It, however, is to be noted that the thickness is desirably 25 mm or less since, when the heat capacity becomes too large, the temperature controllability becomes poor.
If the dispersion of the thickness of a resistance heating element is beyond the range of xe2x88x9270 to +150%, it becomes difficult to make the temperature of a wafer treating face even.
In case where a ceramic substrate is used as an electrostatic chuck, the fact that the ratio (t2/t1) of the average thickness of a conductor layer (t2) to the average thickness of a ceramic substrate (t1) is less than 0.1 means to make the ratio of the thickness of the electrode to the thickness of ceramic substrate small and, therefore, a leakage current at high temperatures between the adjoining conductor layers can be decreased since the area of the side face of the conductor layers becomes smaller. Also, thus, it is possible to reduce the dispersion of apparent resistance value by adjusting the dispersion of the thickness of the electrode to the average thickness of the electrode, to the range of xe2x88x9270 to +150%. As a result, the dispersion of chuck power at high temperature can be reduced.
Moreover, when a ceramic substrate is used as a wafer prober, the fact that the ratio (t2/t1) of the average thickness of a conductor layer (t2) to the average thickness of a ceramic substrate (t1) is less than 0.1 means to make the ratio of the thickness of the electrode to the thickness of ceramic substrate small and, therefore, a leakage current at high temperature between the adjoining conductor layers can be decreased because the area of the side face of the conductor layers becomes smaller.
For this reason, it is possible to reduce the dispersion of apparent resistance value by adjusting the dispersion of the thickness of the electrode to the average thickness of the electrode, to the range of xe2x88x9270 to +150%. As a result, the dispersion of voltage applied to a guard electrode or a ground electrode can be reduced and the generation of noise can be reduced.
The above-mentioned conductor layer desirably has a dispersion of the thickness thereof to the average thickness thereof within the range of xe2x88x9230 to +30%.
JP Kokai Hei 11-251040 discloses a heater having a dispersion of thickness of a resistance heating element of xc2x110%, but there is no description about the thickness of a ceramic substrate.
It is to be noted that: the effect that no warp or no cracks are caused is not expected in the case where the relation of the thickness of a ceramic substrate and the thickness of a resistance heating element are within a specific range as in the present invention and, therefore, the patentability of the present invention is never affected by the cited reference.
Specifically, when manufacturing a ceramic substrate using a green sheet, it is desirable that the dispersion of the thickness of the green sheet to the average thickness of the green sheet is within the range of xe2x88x9210 to +10%, or that a green sheet having a surface with a roughness of 200 xcexcm or less in the value of Rmax is used. Even if the dispersion of the thickness of a green sheet to the average thickness of the green sheet is within the range of xe2x88x9210 to +10%, when the roughness of the surface of the green sheet exceeds 200 xcexcm in the value of Rmax, it is desirable to adjust the roughness within 200 Mm in the value of Rmax.
When a plate-like body or a film-like body such as a metal foil is used, it is desirable that the metal foil and the like has a dispersion of its thickness to its average thickness of xe2x88x9210 to +10%.
The above is the description about a method for disposing a conductor layer to a ceramic substrate. However, the method for disposing a conductor layer to a ceramic substrate of the present invention is not restricted to the above-mentioned method and it is only necessary that the dispersion of the thickness of a conductor layer formed to the average thickness of the conductor is, as a result, within the range of xe2x88x9270 to +150%.
The above-mentioned local dispersion of chuck power of an electrostatic chuck can be judged by the measurement of the surface temperature of the adsorbed semiconductor wafer by a thermoviewer. This is because if a semiconductor wafer is adsorbed firmly to the adsorbing face of an electrostatic chuck, the temperature of the portion adsorbed firmly becomes high and, therefore, the temperature distribution of the semiconductor wafer reflects the dispersion of chuck power.
It is also possible to measure the dispersion by putting a semiconductor wafer which is partitioned into a plurality of partitions and then measuring adsorbing power of the semiconductor wafer in each partition with a load cell. In the following Examples, both of the above-mentioned methods were adopted.
The above-mentioned local dispersion of the amount of the heat generated by a resistance heating element can be judged by measuring the surface temperature of a ceramic substrate or the surface temperature of a semiconductor wafer put on a wafer treating face using a thermoviewer. This is because the local dispersion of the amount of the heat generated is also reflected to the temperature distribution of the wafer treating face.
In this case, it is recommended to dispose only a resistance heating element inside a ceramic substrate and then measure the temperature distribution of a semiconductor wafer.
Alternatively, in the case of wafer probers, the dispersion of applied voltage of a guard electrode or a ground electrode can not be measured directly. Therefore, a continuity test should be conducted using a silicon wafer that has been judged to be an acceptable product which is commercially available. In this continuity test, if this silicon wafer is judged to be an acceptable product, it should be thought that there is little dispersion.
Next, a description will be made to the structure of the ceramic substrate of the present invention and to materials constituting the ceramic substrate.
The ceramic substrate of the present invention is desirably used at 100xc2x0 C. or higher, and most desirably at 200xc2x0 C. or higher.
In the above-mentioned ceramic substrate, the pore diameter of the maximum pore is desirably 50 xcexcm or less. The porosity thereof is desirably 5% or less. It is also desirable that no pores are present or, if present, the pore diameter of the maximum pore is 50 xcexcm or less.
If no pores are present, breakdown voltage becomes especially high at high temperature. Conversely, if pores are present to some extent, fracture toughness becomes higher. Thus, which is designed depends on required properties.
The reason why fracture toughness becomes higher due to the presence of pores is unclear, but the inventors presume that the reason is based on the stop of development of cracks by the pores.
The reason why the pore diameter of the maximum pore is desirably 50 xcexcm or less in the present invention is that if the pore diameter is over 50 xcexcm, it is difficult to keep high breakdown voltage property at high temperature, particularly at 200xc2x0 C. or higher.
The pore diameter of the maximum pore is desirably 10 xcexcm or less. This is because the warp amount becomes small at 200xc2x0 C. or higher.
The porosity and the pore diameter of the maximum pore can be adjusted by pressing time, pressure and temperature at the time of sintering, or additives such as SiC and BN. Since SiC or BN hinders sintering, pores can be produced.
When the pore diameter of the maximum pore is measured, five samples are prepared. The surfaces thereof are ground into mirror planes. With an electron microscope, ten points on the surface are photographed with 2000 to 5000 magnifications. The maximum pore diameter is selected from each of the photos obtained by the photographing, and the average of the 50 shots is defined as the pore diameter of the maximum pore.
The porosity is measured by Archimedes"" method. This is a method comprising the steps of crushing a sintered product to pieces, putting the pieces into an organic solvent or mercury to measure the volume thereof, obtaining the true specific gravity of the pieces from the weight and the volume thereof, and calculating the porosity from the true specific gravity and apparent specific gravity.
The diameter of the ceramic substrate of the present invention is desirably 200 mm or more. It is especially desirable that the diameter is 12 inches (300 mm) or more.
In a large substrate having a diameter exceeding 150 mm, the thickness of an electrostatic electrode or resistance heating element is set so that the dispersion of its thickness to its average thickness is within the range of xe2x88x9270 to +150% as described above, otherwise the dispersion of the temperature of a heated semiconductor wafer will become big due to the dispersion of chuck power. In addition, the heat capacity of the ceramic substrate will increase, resulting in unevenness of the temperature of the wafer treating face. Conversely, in a substrate with a diameter of about 150 mm, the surface temperature becomes even, even if there is some dispersion of chuck power because the semiconductor wafer treated thereof is also small. Furthermore, even if there is some dispersion of the resistance value of a resistance heating element, the temperature of a ceramic substrate rises easily and the temperature easily becomes relatively even since the ceramic substrate has a small heat capacity.
The thickness of the above-mentioned ceramic substrate is desirably 50 mm or less, and especially desirably 25 mm or less.
This is because, if the thickness of the ceramic substrate exceeds 25 mm, the heat capacity of the ceramic substrate may be too large, and particularly when a temperature control means is set up to heat or cool the substrate, the temperature following property may become poor because of the large heat capacity.
The thickness of the ceramic substrate optimally exceeds 1.5 mm and is 5 mm or less.
In the case where the thickness of the ceramic substrate is 1.5 mm or less, if the ceramic substrate is such a large one that its diameter exceeds 150 mm, it will warp too much and, therefore, is poor in practical utility.
The ratio (t2/t1) of the thickness of the conductor layer (t2) to the thickness of the ceramic substrate (t1) is adjusted to be less than 0.1. The ratio is more desirably less than 0.01. This is because less ratio of the thickness of the conductor layer to the thickness of the ceramic substrate results in less occurrence of crack or warp, in less leakage current at high temperature and in less dispersion of the temperature on the wafer treating face.
The ceramic material constituting the ceramic substrate is not especially limited. Examples thereof include nitride ceramics, carbide ceramics, oxide ceramics and the like.
Examples of the nitride ceramics include metal nitride ceramics such as aluminum nitride, silicon nitride, boron nitride and the like.
Examples of the carbide ceramics include metal carbide ceramics such as silicon carbide, zirconium carbide, tantalum carbide, tungsten carbide and the like.
Examples of the oxide ceramics include metal oxide ceramics such as alumina, zirconia, cordierite, mullite, beryllia and the like.
These ceramics may be used alone or in combination of two or more thereof.
Among these ceramics, the nitride ceramics and oxide ceramics are preferred.
Aluminum nitride is most preferred among the nitride ceramics since its thermal conductivity is highest, that is, 180 W/mxc2x7K.
The ceramic substrate preferably contains 0.05 to 10% by weight of oxygen. This is because the fracture toughness can be improved by segregating oxygen in grain boundary.
This is also because if the oxygen content is less than 0.05% by weight, the sintering does not proceed, resulting in an increase in the porosity and a drop in the thermal conductivity, and contrarily, if the oxygen content exceeds 10% by weight, the thermal conductivity drops due to too much oxygen in the grain boundary, resulting in the deterioration of the temperature-rising property and the temperature-falling property.
In order to incorporate oxygen into the above-mentioned ceramic substrate, ingredient powder is fired in an oxidizing atmosphere or a metal oxide is mixed with ingredient powder and then the mixture is fired.
Examples of the metal oxide include yttria (Y2O3), alumina (Al2O3), rubidium oxide (Rb2O), lithium oxide (Li2O) and calcium oxide (CaCO3) and the like.
The content of these metal oxides is preferably from 0.1 to 20% by weight.
In the present invention, the ceramic substrate preferably contains 5 to 5000 ppm of carbon.
This is because the ceramic substrate can be blackened by the incorporation of carbon and when the substrate is used as a heater, radiation heat can be sufficiently used.
The carbon may be either amorphous one or crystalline one. This is because the use of amorphous carbon can prevent a drop in a volume resistivity at a high temperature and the use of crystalline carbon can prevent a drop in heat conductivity at a high temperature. Accordingly, depending on the purpose, both crystalline carbon and amorphous carbon may be used together. The content of carbon is desirably 50 to 2000 ppm.
When the ceramic substrate is caused to contain carbon, the ceramic substrate is desirably caused to contain carbon so as to have a brightness of N6 or less based on the provision given in JIS Z 8721. This is because a ceramic substrate having such brightness is superior in radiation heat capacity and covering-up property.
Herein, the brightness N is defined as follows: the brightness of ideal black is made to 0; that of ideal white is made to 10; respective colors are divided into 10 parts in the manner that the brightness of the respective colors is recognized stepwise between the brightness of black and that of white at equal intensity intervals; and the resultant parts are indicated by symbols N0 to N10, respectively.
Actual brightness is measured by comparison with color chips corresponding to N0 to N10. One place of decimals in this case is made to 0 or 5.